(a) Field of the Invention
The present invention relates to a zoom lens system having a large aperture ratio of F/1.2 to F/1.4 and a zooming ratio on the order of 8 for use with television cameras.
(b) Description of the Prior Art
Under the present circumstance, portable television cameras adopt mainly image pick-up tubes having a faceplate diameter of 2/3 inch, and photographing zoom lens systems for these cameras generally designed for F numbers of 1.4 to 2.0 and zooming ratios of 2 to 6. Along with the recent compact design of television cameras, however, there is a trend to manufacture image pick-up tubes having a size of 1/2 inch, and compactness is demanded also for photographing lens systems.
When faceplate diameter of image pick-up tubes is reduced from 2/3 inch to 1/2 inch, sensitivity of said image pick-up tubes is lowered, thereby making it necessary to increase aperture ratio of the lens systems for use with said image pick-up tubes by reducing F number of 1.2 to 1.4. Further, when magnification at the stage of image reproduction on CRT is taken into consideration, it is necessary to enhance resolution on the photographing surface. Therefore, it is not easy to design a more compact photographing lens system when faceplate diameter is reduced from 2/3 inch to 1/2 inch.
As a conventional example of a photographing lens systems compatible with the image pick-up tubes having the size of 1/2 inch, there has already been known the one disclosed by Japanese Published Unexamined Patent Application No. 102208/83. This zoom lens system has a total length of approximately 2.1 times of its focal length at the tele position, an F number of 1.2 even at the tele position, a varifocal ratio of 6, compact design and high performance. However, this zoom lens system has a defect that its back focal distance is too short to interpose shutter and other members between the lens system and image pick-up tube.